Fibroblast cell strains from normal newborns and from patients with Down's syndrome were grown in monolayer cultures and their transformation frequencies were determined in response to KiMSV(KiMuLV). The strains with chromosomal abnormalities showed a significantly higher transformation frequency than did normal diploid fibroblasts. However, the majority of normal cell strains reverted to cell strains with normal growth behavior after propagation in-vitro. Only the virus-infected fibroblasts from patients with Down's syndrome acquired neoplastic characteristics, as judged from growth in agar and transplantability into heterologous hosts.